1. Field of the Invention
This invention relates to a novel process for imparting thixotropy to unsaturated polyester liquid resins. More specifically a gelling grade clay and a surfactant are mixed into the polyester resin to form a thixotropic composition, the surfactant being intimately mixed into the clay by blending, without making provision for chemically reacting clay and surfactant.
2. Description of the Prior Art
Typical unsaturated liquid polyesters are polyesters of a dicarboxylic acid, e.g. maleic, phthalic or their anhydrides, and a diol, e.g. ethylene glycol, propylene glycol. These polyesters are made by conventional techniques of esterification well known in the art. Other polyesters may be made by varying the components or the manufacturing process. Commerical polyester systems commonly contain, in widely varying amounts, an unsaturated aromatic monomer having one or more ethylenically unsaturated radicals in addition to the polyester. This monomer is useful as a cross-linking agent and as a diluent to reduce viscosity. Styrene is the most effective and widely used aromatic monomer of this type.
In the application of liquid polyester resins thixotropy is a desirable characteristic. It prevents running or sagging after the material is applied. In highly filled systems such as automotive body patch thixotropy prevents separation of the solid filler particles from the liquid resin by gelling the system into a semi-solid state. This prevents the particles from floating or sinking under the action of buoyant and gravitational forces. A thixotropic system is one that forms a solid-like gel upon standing, but becomes liquid under mechanical agitation and flows easily under shear. The thixotropic system is reversible, i.e. once the liquid has flowed and shear is removed, a gel structure immediately begins to reform in the system. Viscosity begins to increase until the solid-like gel is formed, and the material will not flow under the action of gravity.
Pyrogenic or fumed silica supplied as a fine, fluffy powder is often used commercially to impart thixotropy to liquid polyester resins. Silica is expensive compared to thickening agents comprising gelling grade clays. It also has further drawbacks of settling and loss of sag control on short term aging. Clays may be adapted for use in polyester systems, but they are often required in larger amounts than the silica. When clay is added in excessive amounts, the resin loses its transparency. It may also become tinted. In addition clays may require more energy for mixing than the silicas. If the clay is surfactant modified, the surfactant must be one that will not interfere with the activators and catalysts that are used to cure the resins.
Gelling grade clays such as the expanding or swelling montmorillonites, including bentonite and hectorite, and the needle-like attapulgite, also known as Georgia-Florida fuller's earth, may be adapted to polyester resin systems by modifying the surface to make it more organophilic. This modification may be carried out by (a) chemically reacting an appropriate surfactant, either cationic or nonionic, with the clay to form so-called "onium-clays", "organo-clays" or clay-surfactant reaction products, or (b) physically sorbing or coating an appropriate surfactant on the clay particles by mixing or blending them together. In the case of the blended product the third component, generally the liquid to be thickened, may be added to the clay first or to the surfactant first, the other component being blended in thereafter. In either case moderately high shear mixing is usually required for complete dispersion. If mixing is carried out for too long a time or at shear rates too high, no gelling will take place. If mixing is carried out for too short a time or at shear rates too low, incomplete mixing and poor gelling will result. One formulation of clay and surfactant will in general not be adequate to thicken a system other than the one it was adapted for. On the other hand several different formulations of clay and surfactant may be adequate to thicken a given system.
In thickening polyester resins U.S. Pat. No. 3,795,650 to Burns discloses the use of a polyol and a gelling grade clay (attapulgite, montmorillonite, or sepiolite) worked into a pregel formulation with the resin, whereupon the pregel is then used to impart thixotropy to so-called hand layup moldings. U.S. Pat. No. 3,974,125 to Oswald et al discloses a surfactant-clay reaction product, where the surfactant, a dimethyl-di (hydrogenated tallow) ammonium chloride, is bonded to a montmorillonite clay by chemical reaction. The organo-clay is used to produce a thixotropic polyester pregel (with styrene) which is then used to thicken a wide variety of polyester systems. U.S. Pat. No. 4,081,496 to Finlayson discloses another surfactant-clay reaction product in a pregel formulation. Smectite-type clays such as bentonite and hectorite are reacted with certain types of quaternary ammonium compounds such as a methyl benzyl dialkyl ammonium chloride in specific proportions to produce the organo-clay. This clay gellant is mixed with an unsaturated aromatic monomer, such as styrene, under high shear to form a soft pregel.
Modifying clay with a surfactant for the purpose of gelling a liquid system is well known in the prior art. For example, U.S. Pat. No. 2,797,196 to Dunn et al shows a modified clay for use in oil-base drilling fluids consisting essentially of anhydrous clay wetted with a non-anionic surfactant such as a quaternary ammonium salt, and an oily pugging agent. In the grease art, U.S. Pat. No. 2,819,210 to Haden et al discloses a two-surfactant system formed by a tertiary amine having an alkyl group of twelve to eighteen carbon atoms and two polyoxyethylene groups attached to the nitrogen and an alkylammonium salt of a monoalkyl alkylamidophosphate. These surfactants are mixed with an attapulgite clay of 5-30% moisture content and a hydrocarbon oil to form a bodied lubricant or grease. U.S. Pat. No. 2,885,360 to Haden et al discloses a similar composition for bodying a wide variety of organic liquids. A tallow fatty dialkyl dimethyl quaternary ammonium chloride was used in a process in which water was vaporized from the clay during dispersion of the constituents in the organic liquid. Ferrigno U.S. Pat. No. 2,975,071 discloses a method for making gelled thixotropic paints by mixing into an alkyd vehicle a polyisocyanate and a colloidal clay. Miericke U.S. Pat. No. 3,027,265 discloses as onium clay reaction product formed by metathesis (a chemical reaction involving the exchange of ions; reference is made to Hauser U.S. Pat. No. 2,531,427) for use in molding sands. The organophilic clay is mixed with an organic liquid lubricant to form a thixotropic binder for the sand. Sawyer U.S. Pat. No. 4,147,519 discloses a method for thickening suspension of coal dust in organic liquids by adding a gelling grade clay and an organic cationic or nonionic surfactant.
An object of the present invention is to formulate a thixotropic polyester composition in which clay and surfactant will be an economic replacement for pyrogenic silica as a thixotrope for polyester resin systems, especially those in which haze or color tint are not important. Another object of the invention is to formulate a thixotropic polyester composition containing clay that will retard separation of filler and liquid resin in filled polyester systems. Still another object of the invention is to formulate a thixotropic polyester composition containing clay to prevent sags and runs in liquid polyester resins after being applied. An additional object of the invention is to formulate without pregel a thixotropic polyester composition containing clay that will be effective in thickening high viscosity or highly filled polyester resins.